A projecting system is widely used in our daily lives to project texts/pictures or image data onto a screen in order to facilitate many persons to view the enlarged texts/pictures or image data in a visually comfortable manner. Recently, the general trends in designing electronic devices are toward small size, light weightiness and easy portability to meet the requirements of humanization. Similarly, the projecting system is developed toward miniaturization. As a consequence, the projecting system can be applied to electronic devices such as 3G mobile phones or personal digital assistants (PDAs). In addition, a small-sized projecting system becomes a portable electronic device. On account of its portability, the user may use small-sized projecting systems to watch movies everywhere they are, and thus the amusement purpose can be achieved without difficulty.
FIG. 1 is a schematic view illustrating a conventional projecting system. As shown in FIG. 1, the conventional projecting system 1 comprises a color light mixing device 11, a prism 12, a microdisplay element 13 and an optical projection lens assembly 14. For example, the microdisplay element 13 is a digital micromirror device (DMD). The light source provided by the color light mixing device 11 illuminates the microdisplay element 13 through the prism 12. Consequently, the image shown on the microdisplay element 13 is transmitted through the prism 12 and then directed to the optical projection lens assembly 14. The image is processed by the optical projection lens assembly 14 and then projected onto a screen 9 in front of the optical projection lens assembly 14, so that the image is enlarged. As known, if the light beams projected on the microdisplay element 13 are not homogenized, the brightness level or color at the border or the middle region of the image may be non-homogeneously distributed. That is, the quality of the light beams outputted from the color light mixing device 11 is one of the most important factors that influence the imaging performance of the projecting system 1.
FIG. 2 is a schematic view illustrating a color light mixing device according to the prior art. As shown in FIG. 2, the color light mixing device 11 comprises a red light-emitting unit 111, a green light-emitting unit 112, a blue light-emitting unit 113, a light combination unit 114 and a tube structure 115. By the light combination unit 114, the red light beams outputted from the red light-emitting unit 111, the green light beams outputted from the green light-emitting unit 112 and the blue light beams outputted from the blue light-emitting unit 113 are combined together. The combined light beams are directed into the tube structure 115. The combined light beams are reflected or scattered within the tube structure 115 for several times, and thus mixed light beams with more homogeneous brightness level or color are outputted from the tube structure 115.
The conventional projecting system 1, however, still has some drawbacks. For example, since the light beams outputted from various light-emitting units need to be combined together by the light combination unit 114 of the color light mixing device 11, the configuration of the color light mixing device 11 is complicated. Consequently, the overall volume of the conventional projecting system 1 fails to be effectively reduced.
Generally, the homogenizing degree of the brightness level or color of the light beams is positively directed to the length of the tube structure 115. That is, as the length of the tube structure 115 is increased, the brightness level or color of the light beams is distributed more homogeneously. However, if the tube structure 115 is too long, the loss of the light energy is increased and the overall volume of the projecting system 1 fails to be effectively reduced. On the other hand, although the overall volume of the tube structure 115 can be reduced by shortening the length of the tube structure 115, the homogenizing efficacy of the mixed light beams becomes unsatisfied. In other words, the use of only the tube structure 115 is insufficient to homogenize the brightness level or the color of the light beams.
Therefore, there is a need of providing an improved color light mixing device to obviate the drawbacks encountered from the prior art.